The invention relates to rolls and dryer groups in a paper or board machine.
The use of double fabric run and/or single fabric run arrangements in the dryer groups of multi-cylinder dryer sections of paper or board machines is previously known. In the double fabric run arrangement the dryer cylinder groups have two fabrics pressing the web one from above and the other from below against heated cylinder surfaces. Between the dryer cylinder rows, generally horizontal rows, the web has, in the double fabric run arrangement, free and unsupported draws, which are liable to fluttering, which may cause web breaks, particularly at such drying stages, in which the web is still relatively moist and therefore weak for its strength. Due to this, during several past years there has been increasing use of the said single fabric run arrangement, in which each dryer cylinder group has only one dryer fabric, supported by which the web travels through the entire group, the dryer fabric pressing the web against the cylinder surfaces heated by the dryer cylinders and the web remaining outside the outer curve at the turning cylinders or turning rolls located between the dryer cylinders. Thus, in the single fabric run arrangement, the dryer cylinders are positioned outside the fabric loop and the turning cylinders or turning rolls are located inside the loop. In so called normal single fabric run groups the dryer cylinders are in the top row and the turning cylinders or turning rolls are in the bottom row, and correspondingly, in so called turned single fabric run groups, the dryer cylinders are in the bottom row and the turning cylinders or turning rolls are in the top row.
In known dryer groups adapting the single fabric run arrangement the dryer fabric and the paper web are transferred from the previous drying element, such as a contact dryer cylinder, to a turning or suction cylinder or similar in a common straight run, whereby a closing wedge space, also referred to below as closing nip, is formed between the dryer fabric and the last-mentioned turning cylinder or suction cylinder surface. The dryer fabric and the cylinder surfaces moving towards this nip tend to generate positive pressure in the said wedge space by means of the boundary layer flows conveyed by them. This again produces a pressure difference over the paper web supported by the dryer fabric, the pressure difference having a tendency to detach the paper web from the dryer fabric causing runnability problems, wrinkles, and even web breaks. On the other hand, for improving the efficiency of dryer sections, the need arises for using dryer sections with a more compact construction than heretofore, in which the contact dryer cylinders and the mentioned suction cylinders are as close as possible to each other. All these aspects together with rising web speeds increase the overpressure problems of the said closing nip. It is previously known that the transfer of the paper web in the single fabric run arrangement on the contrary from the turning suction cylinder to the contact dryer cylinder takes place after a so called opening nip, supported by the dryer fabric. In dryer sections suitable for the single fabric run arrangement, the term pocket space is used to refer to the pocket-like space, which is limited by two parallel dryer cylinders and the turning cylinder and dryer fabric between them.
In the solutions known in the prior art technique, attempts have also been made to remove the problems occurring in the area of the closing nip by means of roll suction, roll sector suctions and various types of vacuum-generating boxes as well as by using combinations of rolls and suction boxes, which, however, have not necessarily been able to completely eliminate the problems in this area in an energy-efficient manner. At high machine speeds the requirement of energy used for web stabilization also strongly increases. Typically the power requirement increases to the power of three in relation to the web speed.
A solution for removing the problems in this area is set forth in the FI patent No. 105573 (corresponding U.S. Pat. No. 5,996,244), which discloses a roll in a paper machine, particularly in a paper drying device, and a dryer group in a paper machine, in which the roll in the paper drying device comprises a shaft, supported by which the roll is adapted to rotate, and a surface structure, connected to the shaft with support pieces or similar, in which the openness of the surface structure of the roll is more than 10% and the surface structure of the roll is open in a slot-like manner so that during the roll rotation an effect is produced that aspirates air to inside the roll, whereby an air flow-through is created through the roll. In a dryer group of a paper machine, in which dryer group the single fabric run arrangement is adapted, at least one of the turning rolls of the dryer group is an open roll of the type described above.